Poly(phenylene ether)s are a class of plastics known for excellent water resistance, dimensional stability, and inherent flame retardancy, as well as high oxygen permeability and oxygen/nitrogen selectivity. Properties such as strength, stiffness, chemical resistance, and heat resistance can be tailored by blending poly(phenylene ether)s with various other plastics in order to meet the requirements of a wide variety of consumer products, for example, plumbing fixtures, electrical boxes, automotive parts, and insulation for wire and cable.
Due to its widespread use, improvements in the method of making poly(phenylene ether)s remains an active area of research. In general, poly(phenylene ether)s are synthesized by oxidative polymerization of a sterically hindered phenol in the presence of a poly(phenylene ether) solvent, a catalyst metal ion, and at least one catalyst amine ligand. In general, a poly(phenylene ether) reaction mixture thus includes not only the desired poly(phenylene ether), but also poly(phenylene ether) solvent, catalyst metal ion, and catalyst amine ligand, as well as colored impurities and odorous impurities. When the poly(phenylene ether) is isolated from the reaction mixture, catalyst metal ion, catalyst amine ligands, colored impurities, and odorous impurities must all be reduced to acceptable levels. If the colored impurities and the catalyst metal ions are not removed from the poly(phenylene ether), it will suffer from poor thermal and oxidative stability, and it will discolor during melt blending, extrusion, and molding above about 250° C.
One purification method for poly(phenylene ether)s comprises precipitation of solid poly(phenylene ether) from a solution in the poly(phenylene ether) solvent, for example toluene, with an antisolvent for poly(phenylene ether), for example methanol. In this method, the precipitated poly(phenylene ether) is filtered, washed with antisolvent, and optionally reslurried with antisolvent, filtered, and washed again. Residual antisolvent is then removed by drying. Drawbacks to this method are the capital costs of solids handling equipment, the relatively high cost for the processing of solid poly(phenylene ether), and the large volumes of antisolvent that are required.
The catalyst metal ion, for example copper ion, can be removed by washing the poly(phenylene ether) solution in the poly(phenylene ether) solvent with an aqueous solution of a chelating agent. See, for example, U.S. Pat. No. 3,838,102 to Bennett. U.S. Pat. No. 4,237,265 to Eliassen et al. discloses a cocurrent or countercurrent liquid-liquid extraction in an extractor such that a continuous aqueous phase is maintained while a discrete poly(phenylene ether) solution phase is continuously contacted by the aqueous phase. The aqueous phase is a mixture of water and an alkanol having 1 to 4 carbon atoms. This process requires an a relatively large volume of aqueous phase relative to the poly(phenylene ether) solution phase, in particular a volume ratio of aqueous phase to poly(phenylene ether) solution phase of 1:1 to 10:1, with a chelating agent in the aqueous phase. This method has at least two disadvantages. First relatively large amounts of the aqueous phase comprising water and alkanol are used relative to the amount of poly(phenylene ether) produced, which means that a large amount of alkanol is used. This high alkanol usage adds to the cost of the process. Also, since the aqueous phase contains a large amount of a flammable alkanol, the alkanol must be removed from the water before the water can be disposed of. For example, when the alkanol is methanol, the methanol must be removed from the waste water by distillation, a highly energy-intensive process.
Another approach to catalyst removal is disclosed in U.S. Pat. No. 4,654,418 to Berger et al. The poly(phenylene ether) solution is combined with an aqueous chelating agent solution in a mixing step, sent to a settler where the phases are separated, and the process is repeated with additional aqueous chelating agent solution. The aqueous chelating agent solution is recycled from the second step to the first step, which improves the overall organic to aqueous phase ratio to 1.0:0.1 to 1:1.0. However, since it is a continuous process, two mixing tanks and two settling tanks are required. Moreover, Berger is silent as to the effectiveness of the process to remove colored impurities from the poly(phenylene ether).
Another approach to catalyst removal is disclosed in U.S. Pat. No. 6,576,738 to Braat et al. Braat describes a process for removal of the copper catalyst from a poly(phenylene ether) solution by adding at least one polar solution to form a two phase mixture. The two phases are separated using a liquid/liquid centrifuge. A polar solvent is added to the separated poly(phenylene ether) solution to form a second two phase mixture, which is also separated using a liquid/liquid centrifuge. The resulting poly(phenylene ether) solution has a copper content of less than about 1.1 milligrams per kilogram (1.1 ppm). Due to a low density difference between the aqueous phase and the poly(phenylene ether) solution, a liquid/liquid centrifuge is required to achieve phase separation. A related approach to catalyst removal is disclosed in U.S. Patent Application Publication No. US2007/0299243 A1 to Delsman et al. In Delsman, a poly(phenylene ether) solution and a chelating agent solution are also separated in a liquid/liquid centrifuge. However, the centrifuge residence time is less than or equal to 60 seconds. Two aqueous washes, the first comprising a chelating agent, are required to obtain copper levels of less than 1 milligram per kilogram (1 ppm). Copper levels of less than milligram per kilogram (1 ppm) could not be achieved by gravity separation of the poly(phenylene ether) solution and aqueous washes. Delsman is silent as to the effectiveness of the process to remove colored impurities from the poly(phenylene ether).
There remains a need in the art for a poly(phenylene ether) purification process which overcomes the problems associated with precipitation of solid poly(phenylene ether) with an antisolvent, i.e. the capital costs of solids handling equipment, the relatively high cost for the processing of solid poly(phenylene ether), and the large volumes of antisolvent that are required. There also remains a need in the art for a poly(phenylene ether) purification process which reduces the amount of antisolvent used. It is desirable that the process not only removes catalyst metal ions, but also removes colored impurities, thereby improving both the color, and the thermal and oxidative stability, of the isolated poly(phenylene ether).